The object of this invention is a method by which arsenic is removed from sulfuric acid solution, of which the sulfuric acid content is at least 300 g/l at a temperature of 50-105xc2x0 C., by reducing the arsenic of the solution with the aid of sulfur dioxide. The method is particularly suitable to be used together with electrolytic refining of metals, such as in copper refining.
In principle, the impurities present in electrolytically leaching of anode copper behave in two different ways: impurities more basic than copper dissolve and the more noble remain undissolved, whereupon they form so-called anode slime. In both cases there is a possibility as a result that the impurities precipitate onto the cathodes and thus cause contamination in cathode quality.
Arsenic occurs in the anode together with other elements as various compounds, of which the electrochemical potentials differ from each other, whereupon part dissolves in the electrolyte and part falls into the anode slime. The composition of the anodes and electrolysis conditions affect the distribution of these impurities between the electrolyte and anode slime.
Arsenic dissolves into the electrolyte as trivalent ion. As a result of different oxidation reactions, arsenic occurs, however, as pentavalent in the electrolyte. If soluble arsenic is not removed from the electrolyte, it gradually accumulates in the electrolyte. It is not, however, absolutely necessary to completely remove arsenic from the solution: it is sufficient to keep its content below a certain level.
The modern processes for the removal of arsenic in use in copper electrolysis are based, for the most part, on the electrolytic reduction of arsenic in so-called decopperizing electrolysis or on the utilization of extraction technology.
Decopperizing electrolysis technology is generally incorporated with nickel removal from the electrolyte. When nickel is removed from the electrolyte, copper is first removed with the aid of decopperizing electrolysis using insoluble lead anodes. In connection with decopperizing electrolysis it is at the same time possible to remove arsenic as copper arsenide at the final stage of the process, as the copper content of the solution is reduced sufficiently low. From the copper-free solution, nickel is obtained by concentrating the solution. The process has the advantage of being old, well-known technology. The disadvantages of the process are especially when it is used for the removal of arsenic, the possibility of forming of toxic arsine gas in the final stage of the process and the disposal of the copper arsenide deposits formed as the end product of the process. When arsenic is removed by this process, copper must also always be removed.
In the art it is known a method where the decopperizing process is based on the use of periodic reversal current technology (PRC) connected with a large solution flow rate and an elevated temperature in the last stage of decopperizing electrolysis. It is thus possible to remove arsenic efficiently without the formation of arsine gas. The process is, however, furnished with an expansive air conditioning and analyzing system in order to ensure the detection and removal of any possible arsine produced. It is possible to remove approximately 80% of the arsenic fed into the last stage of the decopperizing process and the precipitate obtained is copper arsenide.
In Japanese patent application 59-074245 it is described a method where the arsenic of the electrolyte is also removed in decopperizing electrolysis, whereby copper arsenide precipitate is obtained. The copper arsenide precipitate obtained is leached by the aid of oxygen and sulfuric acid and the copper sulfate produced is crystallized and removed from the solution. By cooling the solution, lead sulfate is removed, after which bismuth is removed. Finally the arsenic of the solution is reduced by sulfur dioxide and the arsenic can be precipitated as arsenic trioxide.
The arsenic extraction processes of the electrolyte operating on the commercial scale are based on the use of tributyl phosphate as the extraction agent. In the extraction process arsenic can be obtained as a diluted arsenic acid-sulfuric acid solution, out of which arsenic is to be precipitated. The problem with the process is that the antimony and bismuth contained in the electrolyte precipitate at the extraction stage, and the precipitation produced thereby disturbes the process. The extraction agent may also cause problems in the growth of the cathode in the actual copper electrolysis process.
The best-known electrolyte extraction process is the MHO extraction process, where undiluted tributyl phosphate is used as the extraction agent. Before extraction, copper is removed from the electrolyte while the acid content and at the same time the arsenic content are increased in decopperizing electrolysis and in evaporation. The extraction of arsenic takes place in a separate extraction unit, which simultaneously acts as the concentrator for the removal of antimony-bismuth-arsenic deposition from the process. The subsequent solution after extraction is taken to nickel crystallization. Arsenic is precipitated from the stripping solution as arsenic trioxide, which requires the reduction of arsenic from pentavalent to trivalent.
It is also known in the art another extraction process, which is also based on the use of tributyl phosphate as the extraction reagent. In that case tributyl phosphate is used 50% diluted in Shellsol 2046. The solution for the process is taken from the decopperizing electrolyte, whereby the copper content of the solution has been reduced to a value of about 30 g/l. Then it has also been possible to raise the acid content of the solution. The extraction is based on conventional mixer-settler technology. The arsenic-containing stripping solution obtained is neutralized and the arsenic present is precipitated as copper arsenate. The copper arsenate is sold for the manufacture of wood impregnation material.
According to the method of this invention, the problems laid out in the previous method are overcome. Arsenic can be removed simply and selectively from the electrolyte of a copper electrolysis, from which most of the copper has first been removed. Arsenic is reduced in a concentrated sulfuric acid solution and the only reagent used in the method is sulfur dioxide gas, which reacts to sulfuric acid in the process, which for its part has no harmful effects on the electrolysis plant. Arsenic trioxide obtained is crystallized from the solution by cooling and the resulting product is so pure that it can be used directly, for example in the manufacture of wood impregnation material. The essential features of the present invention are set out in the claims attached.